PYROMETRY 467 1 0 one coordinate and time as the other coordinate. Such a temperature-time curve has been called an autographic record. This type of curve is the most easily obtained mechanically and is valuable as a continuous record of the temperature of a furnace over an extended interval of time. It may be employed to detect transformation points in steel, which appear as flexures or indentations on the plot when the furnace containing the sample of steel is uniformly heated or cooled. For this latter work, however, the "differential temperature" curve recorder is especially adapted and will be described later. The various special methods have been discussed by Burgess.1 A recorder for obtaining a temperature-time curve consists essentially of a deflec-tional instrument with a mechanism for periodically recording the position of the deflecting member upon a chart which moves with a uniform speed. As in the case of simple indicators there are two general types oT recorders, one operating on the galvanometric principle, and one operating upon the potentiometric principle. Also, as in the case of indicating instruments the potentiometric principle while somewhat more complicated has the especially desirable feature that its indications are independent of the thermocouple resistance.2 There are three types of record paper in general use, the roll charts, the drum charts and the disk or circular charts. The roll chart may contain enough paper to last a month or more, while usually the drum or circular charts are renewed every 24 hr. For single-point recorders all of these record forms are employed, but with one or two exceptions multiple-point recorders use record paper in the roll form. Upon circular Fro. 31.óRecording device for thermocouple. charts the lines of equal temperature are represented by concentric circles and lines of equal time by arcs following the course of the galvanometer needle (see Fig. 81). The drum and roll records are the best in point of available width of scale, and the latter is the most widely applicable for recording rapid changes of temperature, because the time coordinates on the drum chart and particularly on the disc chart are very short. The coordinates of the roll and drum charts can be made rectangular but in some cases the lines of equal time are portions of circles the radii of which are the length of the galvanometer needle. Thus in Fig. 31 the lines of equal times arc arcs parallel to the chopper bar B. The record may be obtained by pen and ink, stylus and inked ribbon, inked thread, carbon paper, or coated paper; by puncturing the paper by means of an electric spark; or by stamping upon the record sheet some imprint from a stencil. The various methods have been developed with a view to making more distinct the records of multiple point recorders. The recording millivoltmeter or galvanometer may employ the same galvanometer system used in the portable indicating instruments. A more rugged instrument is desired, however, and is obtained by increasing the strength of the springs, boom, coil, and the pivots and bearings of pivot instruments. To compensate for the resulting decrease in sensitivity, the density of magnetic flux may be increased, the number of turns of wire on the moving coil may be increased, and the so-called swamping resis- 1 Bureau of Standards, Sci. Paper No. 99. 8 See paragraphs on Potentiometer, p. 426.